Would 75 lbs of LR plus a protein skimmer rated for 150 gallons be enough filtration for a bimac in a 75 gallon tank? Should I add a refugium?

Well, you are going to definitely need some way of removing waste products beyond that...a power filter or a wet dry system could be used, depending on your preference. Octopus put out a tremendous amount of waste, several times that of a like sized fish!

Wow thats a lot of waste!

So in a 75 gallon tank with a single bimac I will need:
75 LBS LR
A protein skimmer rated for 150 gallons
A Refugium
And a wet/dry?

DAMN!

The refugium is not necessary, but is a nice perk. I might add that a skimmer rated for 150 gallons might be a bit small--the manufacturers have a tendency to exagerate.

I agree on the wet/dry. The live rock may cut it for a reef tank, but an octopus is an ammonia factory.

Dan

Wow thats a lot of waste!

So in a 75 gallon tank with a single bimac I will need:
75 LBS LR
A protein skimmer rated for 150 gallons
A Refugium
And a wet/dry?

DAMN!

So in a 75 gallon tank with a single bimac I will need:

75 LBS LR - Generally octos are not filtered with live rock alone like reef tanks. Live rock can filter small fish and inverts well, but I feel that even 150 lbs in a 75 gallon would need alot of prep (slowly building the ammonia load) to finally be able to handle the large amounts of waste from a fast growing ceph. Since live rock is totally submerged it is not as efficient in converting ammonia as a wet/dry.

A protein skimmer rated for 150 gallons - Since they make so much ammonia, tons of nitrate will soon follow. Skimmers also remove ink from the water and are considered essential for octo keeping.

A Refugium - This is really for additional nitrate removal via nutrient export in macro algae.

And a wet/dry? - The real workhorse in the filtration system.

Ahhh, OK I see now. Thanks Illithid!